1. Field of the Invention
The invention relates to a secondary battery replacement method for replacing one or more secondary batteries in a battery assembly formed by electrically connecting a plurality of secondary batteries in series or parallel if the one or more secondary batteries have degradation (including reduction in the fully charged capacity, acquisition of memory effect, etc.), or if the one or more batteries reach the end of service life or have a failure. More particularly, the invention relates to a method for replacing secondary batteries installed in pure electric vehicles (PEVs) or hybrid engine vehicles (HEVs).
2. Description of the Related Art
Examples of the secondary battery include lead-acid batteries, nickel-cadmium (Ni—Cd) batteries, nickel-metal hydride (Ni-MH) batteries, lithium ion batteries, etc. Each secondary battery has a characteristic that, when exhausted, the battery can be charged by connecting the battery to an external electric power supply and supplying a predetermined current to the battery. Due to this characteristic, secondary batteries have been used in various appliances. For example, secondary batteries are installed in conventional motor vehicles in order to supply power to ignition plugs of engines.
Recently, in many electric vehicles (PEVs) and many generally-termed hybrid vehicles (HEVs) equipped with an internal combustion engine and an electric motor, Ni-MH batteries are adopted as a main electric power source for driving the electric motor, due to the high energy density (i.e., compact storage of energy) and the high output density of the Ni-MH batteries. For use in PEVs and HEVs, battery assemblies are each formed by combining a plurality of unit cells in series or parallel, and are used as a battery pack, in order to supply sufficient power to the electric motor.
With regard to such a Ni-MH secondary battery installed in PEVs and HEVs in the form of a plurality of battery assemblies connected, a long service life is possible if the service condition is appropriate. In normal cases, however, the battery assemblies individually come to have an abnormality or reach the end of service life one at a time due to individual differences of secondary batteries or defects in component parts of secondary batteries. If in a battery assembly there is a secondary battery that has an abnormality or has reached the end of service life, the battery assembly cannot perform its normal functions, and is very likely to cause a system malfunction. Accordingly, Japanese Patent Application Laid-Open Publication No. 2002-15781 discloses a secondary battery replacement method that allows a battery assembly as a whole to deliver its full performance after replacement of one or more of the secondary batteries of the battery assembly.
The replacement method disclosed in Japanese Patent Application Laid-Open Publication No. 2002-15781 is a method for replacing one or more of the secondary batteries that are electrically connected in series or parallel so as to form a battery assembly with replacement batteries if the one or more secondary batteries are found faulty. The replacement method includes the steps of: detecting the voltage of each one of predetermined voltage detection blocks of the secondary batteries forming the battery assembly; determining whether a voltage detection block has any faulty battery on a block-by-block basis; and replacing the secondary batteries of the voltage detection block that includes the battery that is found faulty by the determining step with replacement batteries.
According to the method disclosed in Japanese Patent Application Laid-Open Publication No. 2002-15781, there is no need to replace the whole battery assembly and no need to identify which one of the secondary batteries is a faulty battery. Instead, secondary batteries, including a faulty battery, are replaced in the unit of a voltage detection block provided for detection and control of battery voltage in an existing construction. Thus, it is made possible to perform this method merely by a change in software, without the need to add a hardware arrangement for detecting a faulty battery. Therefore, the method allows a reduction in the cost of replacement batteries and a reduction in the cost related to detection of a faulty battery. Furthermore, if, in this method, secondary batteries of a highest capacity rank in the group of secondary batteries that form a battery assembly are used as replacement batteries, the capacity of the battery assembly will not be restricted by replacement batteries, so that the performance of the battery assembly can be maintained as high as before the replacement.
Particularly in an application as in HEVs where secondary batteries are not fully charged but are normally used with an intermediate amount of remaining capacity, the following problems may arise; (1) due to varied remaining capacities of the secondary batteries of a battery assembly, the discharge operation of the battery assembly is restricted by a battery that has a small remaining capacity, and the charge operation is restricted by a battery that has a large remaining capacity, so that the capacity of the entire battery assembly cannot be fully utilized; (2) due to varied self-discharge characteristics of the secondary batteries of a battery assembly, the discharge operation of the battery assembly is restricted by a battery of a great self-discharge characteristic, and the charge is restricted by a battery of a small discharge characteristic, so that varied remaining capacities will be caused; and (3) due to varied internal resistances of secondary batteries of a battery assembly, for example, in a case where replacement batteries having great internal resistance are incorporated into the battery assembly, determination of a capacity may produce an error with respect to the actual capacity depending on the method of determination adopted, so that the battery assembly may fail to deliver satisfactory performance.
According to a replacement method disclosed in the Japanese Patent Application Laid-Open Publication No. 2002-15781 with regard to a battery assembly of nickel-metal hydride batteries, a replacement battery module that is higher in capacity rank than any one of the battery modules of the assembly that are not to be replaced, that is, a replacement battery module that will be in the top capacity rank when incorporated into the battery assembly, is selected from battery modules transported and stored at low temperature. The not-replaced secondary batteries have a region of low charging efficiency in a region where the quantity of charged electricity is high (high SOC region) as indicated in FIG. 7A, due to aging degradation or memory effect acquired during use. On the other hand, the replacement battery module is a fresh module having a greater charging capacity than the other battery modules, and provides a normal charging efficiency in a region equivalent to the region of low charging efficiency of the not-replaced battery modules. If a battery assembly formed by a combination of secondary batteries having different charging efficiencies as described above is subjected to repeated charge-discharge cycles without any special process, such as an equalizing process or the like, the different charging efficiencies of the batteries result in varied states of charge (SOC). Therefore, when a battery ECU (electronic control unit) computes the SOC of each battery module, the difference in SOC between the fresh battery and the not-replaced batteries becomes great, so that the battery ECU determines “SOC variation abnormality” as a diagnosis in order to protect the battery as indicated in FIG. 7B. There is another possibility of the fresh battery being overcharged in the high SOC region.